The invention relates to thin-film evaporators of the sort made up of an enclosure delimiting a vapor space, of a central driving shaft projecting into the enclosure, and of a rotor, attached to the driving shaft, made up of axially spaced evaporator surface elements coaxial to said shaft and defining on the one hand an evaporation space that is in communication with the vapor space, and on the other hand a heating space separated from the vapor space. When the evaporator is in use the feed or fluid to be evaporated is supplied at a first end of the rotor and at the inner edge of one or more conical evaporator elements of a first evaporating stage so that it thence spreads outwards as a film and reaches the outer edge as a preconcentrate, that is taken up by way of a scoop tube and passes from the tube to one or more consecutively adjacent evaporating surface elements of at least one second evaporator stage and is run off as final concentrate at the second end of the rotor.
Thin-film evaporators of this art as proposed in the prior art (see the British Pat. No. 1,132,640 and the Swedish Pat. No. 184,175) may be so designed that each two evaporator elements are placed with a small distance between them and have a conical form tapering in the same direction to form a heating space therebetween. To the inside they are terminated by a ring that is welded in place, while to the outside they are fixed to a ring having holes through it so as to form a communication between the space between the evaporator elements and the cylindrical heating space of the rotor. Steam is introduced into the heating space via the hollow driving shaft and a space underneath the lowest evaporator element, such steam then making its way through the holes into the space between the closely spaced evaporator element and running back as a condensate into the heating space under the effect of centrifugal force. The feed is sprayed between each two pairs of evaporator elements via a tube with a number of discharge points and it is directed against the evaporating faces of the evaporator element. The feed spreads in the form of a film in an outward direction and is collected at the outer edge in a gutter or trough formed between the evaporating faces. The troughs are joined up in axial alinement by way of a duct, through which the concentrated feed runs past a number of axially alined evaporating elements to a single discharge trough for this first stage of the evaporator. A scoop tube is mounted for use with this discharge trough so as to scoop up the preconcentrate. The scoop tube is connected via a pump placed outside the evaporator so as to draw up the preconcentrate and pass it via a duct back to the evaporator. The preconcentrate is then sprayed onto the evaporator faces of the last evaporator stage, it spreading out again in the form of a film and endings up at the outer edge as the final concentrate in the vicinity of a further scoop tube, via which it is taken up by a further pump. The vapors produced during the process of evaporation may be taken through the space in the rotor, collected in the enclosure and removed through a central connection.
Thin film evaporators of this sort are more specially suitable for highly concentrating very weak concentrate or feeds. However, they do have the drawback that for heating the material it is necessary to have twinned evaporator elements with the necessary amount of space between them and generally speaking only the lower evaporating face of each second evaporator element for the evaporation of the feed. Furthermore, the structure of the rotor as a whole is comparatively involved. The outer rings, on which the evaporator element are mounted, have to be supported, centered and sealed off from each other. For the transfer of the preconcentrate produced at each evaporating face of the first stage, it is necessary to have a separate bypass round the evaporator element of this first evaporating stage in the form of the duct noted, in which the preconcentrate has to rise against the head of liquid. This duct is prone to clog or foul. For the transfer of the preconcentrate to the second stage and the removal of the final concentrate, it is in each case necessary to have a pump, and the pipes joined therewith have to be thermally insulated.
In particular, trouble conditions are likely to crop up in the narrow spaces between one evaporator element and the next, that are placed in ascending succession in relation to the axis of turning. There is not only a formation of condensate in these narrow spaces, which may be readily run off to the outside by the action of centrifugal force, but furthermore gases are evolved on condensation, which gradually collect in the spaces and reduce thermal transfer to an ever increasing extent. Therefore it is necessary to have venting holes at the top or apex to let off such gases into the vapor space. Such a connection between the heating space and the vapor space, and for this reason with the evaporating faces, is undesired in the case of many feeds.
This is the reason that it is only possible to use heating fluids--primarily steam--which do not cause deterioration of the feed or concentrate chemically in respect of its odor or taste. Organic heating fluids may not be used at a high heating temperature. Because of the relatively low heating temperature of steam it is furthermore not possible for substances with a high molecular weight to be distilled. A further point is that evaporation may not take place under high vacuum, as otherwise the gases and the heating steam would be positively sucked into the evaporation space. The outcome of this would then be that a large number of thermally sensitive non-aqueous feeds, that are only able to be distilled under a high vacuum, would not be able to be handled in such an evaporator.
In a further known type of thin film evaporator (see U.S. Pat. No. 2,734,023), the rotor is composed of an array of adjacent toroidal evaporating elements, that are each in the form of a pair of shell-like metal plates turned towards each other. The vapors are drawn off at one open end of the rotor and passed at a boosted pressure by a compressor into the heating space. Such heating using the vapors, means for its part that there is a low upper limit for the heating temperature and the evaporation rate. This evaporator as well only operates in a single stage.
A further known design (see the U.S. Pat. No. 2,894,879) operating in several stages and also based on the thermocompression principle, had two conical and oppositely facing evaporating elements in each sealed-off stage. The feed was sprayed on the two facing faces of each stage, the concentrate was removed at the outside through a scoop tube and transferred by a pump into the next stage, something that was only possible with the use of a complex system of pipes inside and outside the evaporator. The use of vapor as a heat vehicle meant that this evaporator as well might only be run at a comparatively low temperature and with a small temperature differential between the heating space and the evaporating space.